The present invention relates to ice detectors, which detect the presence of supercooled large droplets, as opposed to normally encountered icing conditions with smaller droplets. In many cases aircraft ice protection systems are not designed to deal with supercooled large droplets.
Various ice detectors have been advanced in the prior art, and generally include a probe that is vibrated at a resonant frequency, and which has circuitry that senses changes in frequency due to accretion of ice on the probe.
These ice detectors work well, but cannot distinguish supercooled large droplet icing encounters from smaller droplet icing encounters. For smaller aircraft in particular, which may have deicing boots or other ice protection devices on leading edges of airfoils, supercooled large droplets will tend to accrete on surfaces that are aft of the boots, to a point where control surfaces may be affected. Since the accretion of the ice cannot be removed by the deicing boots, control surfaces that are affected may potentially become inoperable or ineffective, resulting in loss of control of the aircraft. The present invention operates to detect both normal, small droplet icing conditions, as well as the supercooled large droplet conditions such that the two conditions can be distinguished. Aircraft crews can thus be made aware of supercooled large droplet icing conditions so they can take the special actions that these conditions may require.
The present invention comprises an ice detector that will sense the presence of supercooled large droplets (SLD), in order to provide a warning that supercooled large droplet icing conditions are present. Supercooled large droplet icing may uniquely affect the operability of the aircraft. The ice detector is formed with two probes, one of which is constructed and mounted so that it will be affected primarily by supercooled large droplets, which may cause unique icing when encountered, that is, a build-up of ice where it cannot be removed by the aircraft""s ice protection system, while the other one of the probes will collect all sizes of droplets. By determining the ratio of icing rates of the two different probes, the presence of supercooled large droplets, that are primarily causing ice build-up on the one probe, can be determined.
Three forms of the ice detector of the present invention are included. One form utilizes an air dam downstream from one of the probes, and the other of the probes is a stand-alone probe with a relatively small frontal area. The air dam preferably has a wall with a radiused leading edge that is generally perpendicular to the airflow direction and the associated probe is just upstream from the leading edge wall. The frontal area of the air dam is several times that of the probe. The smaller droplets of supercooled water will tend to follow the air stream around the air dam, while the larger supercooled droplets, due to their inertia, will tend to impact the air dam surface or the ice detector probe upstream of it.
The stand alone probe, with a relatively small frontal area, will serve as an efficient collector of all water droplet sizes in the air stream, but the probe in front of the air dam will be biased to collect the larger droplets. By determining the ratio of icing rates of the two probes, which are substantially identical, the presence of supercooled large droplets can be determined.
The air dam will be heated to prevent icing on the air dam surface. The influence of the heat on the air dam will be minimal on the large droplet detector probe in front of the air dam because the airflow is moving away from the upstream detector probe and the heated air does not strike the probe.
A variation is to use two probes of different transverse dimension, that is, one is smaller than the other. In this variation, the functions of the air dam and its associated probe are essentially combined.
In a second form of the invention, a flow channel is made that has converging side walls forming a contracting section leading to a narrow section flow channel, with a first probe located upstream of a second probe. The first probe is mounted at the beginning of the narrow section or slightly upstream in the contracting section. The second probe is mounted in the narrow section, somewhat downstream of the beginning of the narrow section. It is known that the supercooled water droplets entrained in the air entering the narrow section will have different trajectories as a function of their size. The smaller droplets will essentially follow the air stream, but the larger droplets are delayed in responding to changes in airflow direction induced by the channel geometry because of relatively greater inertia. This results in a focusing or heavier concentration of large droplets a short distance downstream of the start of the narrow section of the channel, and the second probe is placed at the region where large droplets are concentrated. This location has an amplified sensitivity to supercooled large droplets. By determining the relative icing rate between the second probe and the first probe, which is located upstream of the supercooled large droplet focus area, the determination of the concentration of supercooled large droplets can be made.
The specific geometry of the flow channel and specific placement of the probes, which are substantially identical, can be optimized using computational fluid dynamics software for parameters such as the airspeed range of the aircraft and the supercooled large droplet diameter threshold desired for detection. It will be recognized by those skilled in the art that the upstream probe may also be located outside of the channel.
The walls can have heaters to prevent icing on the walls, and the converging walls leading to the narrow section of the flow channel have bleed holes similar to those in known total air temperature sensors, to bleed off heated boundary layer air and prevent heaters from influencing the ice detector probes.
In a third form of the invention, two probes of different transverse size are used. The probes are both preferably circular cylinders, but one of the probes is substantially larger in diameter or cross section than the other probe. Probes of different cross section shapes can be used and the dimensions of the probes transverse to the airflow direction and transverse to the longitudinal axis of the probes will be different.
The probes of the third form of the invention are usually mounted on struts, and are subjected to the same air stream conditions. The smaller probe, as shown, is mounted so that the majority of its surface is outside of or away from the influence of any flow disruption caused by the larger probe, which in this form, is mounted upstream from the smaller probe. The ice (droplet) collection efficiency, that is, the percentage of droplets directly upstream which impact and freeze rather than follow the airstream around the probe, for the smaller diameter probe is much higher for small supercooled droplet sizes than is the collection efficiency for the larger diameter probe. As the supercooled water droplet size increases, the collection efficiency of both probes will increase, but the collection efficiency of the larger diameter probe rises much faster. Thus, the difference in the respective ice accretion rate of the two probes will decrease as droplet size increases. Comparing the rate of ice accretion between the probes permits detecting potential supercooled large droplet conditions.
In the third form of the invention, both of the probes have similar thermodynamic properties to reduce or prevent variations in probe surface temperatures under transient environmental conditions and during warm up and cool down cycles associated with operation of probe deicing heaters. The design of the probes is such that it is desirable for each probe to have a mass and thermal capacity similar to the other probe, even though one is smaller size. For example, by making the large diameter probe thinner walled than the small diameter probe, similar mass can be achieved even with substantial differences in transverse dimensions of the probes.
In a fourth form of the invention, a pair of ice detector probes are used for indicating supercooled large droplet icing. One probe is mounted ahead of the leading end of the airfoil shaped airflow deflector, and the second probe is mounted downstream of the trailing end of the flow deflector. The ice detector probes can be of conventional design, as before, and substantially the same size. The probe ahead of the leading end presents a frontal area that serves as an efficient collector of all droplet sizes of supercooled water droplets in the airflow. Larger droplets which have higher inertia will tend to separate from the airfoil on the trailing end, and will strike the second probe or down stream probe. The airfoil shaped deflector is oriented at an angle to the airflow direction to provide the inertial separation of the larger particles which strike the second probe.
For any form of the invention, a comparison of the rate of change of frequency between the probes can be made to distinguish supercooled large droplet conditions, and the rate of change of frequency itself will indicate the severity of the icing conditions, or in other words how rapidly ice is accreting on each probe and thus, the ice accretion rate on the aircraft. Further, the overall change of frequency can be correlated to the amount of ice that has been accumulated on the probes. The ice accumulation on the probes can be correlated with ice accumulation on critical aircraft surfaces such as the wing, engine cowl, and horizontal stabilizers, through analysis, or by wind tunnel tests.
The probes of the present invention can be magnetostrictive ice detector probes using signal conditioning equipment, deicing provisions and software similar to that used with present probes, but the controller or processor used is programmed to provide the ratios of rates of icing between the probes, to determine the presence of supercooled large droplets.
The term large droplet is typically defined as a droplet that is 50 microns in diameter or greater. Droplets that are smaller than 50 microns are small droplets or generally referred to as xe2x80x9cdropletsxe2x80x9d, without using the designation xe2x80x9csmallxe2x80x9d.
Droplet populations as seen in nature, as well as those artificially generated with spray nozzles are made up of a continuum of sizes. The aggregate size of the population is typically characterized by the median droplet diameter, or MVD, which is the diameter at which half the droplets in the population are larger and half are smaller. In practice, a droplet population with an MVD greater than 50 microns is generally considered to be a supercooled large droplet condition. The invention in effect can be tuned to distinguish MVDs of 50 microns in diameter or greater, when sufficient quantities of these large droplets are present.